Compounds that "uncouple" oxidative phosphorylation, viz., NaN3, oligomycin and dinitrophenol, cause microtubules to polymerize in the cytoplasm and nucleus of mast cells but not macrophages and lymphocytes. The cortex of certain cells and the microvilli protruding from it appear to be supported and maintained by a network of fibers consisting principally of actin. The density of microvilli on omental mesothelial cells can be increased or decreased by raising or lowering the ambient protein concentration. Certain types of proteins are more efficient effectors than others. Pits, 150-400 nM in diameter, appear on the cell surface after exposure to protein. The pits, which follow kinetics similar to microvilli, are most frequently found at the base of a microvillus.